However, few have actually been studied in enough detail to know if they select which prey to eat.

So Dr Jerome Spitz and colleagues at the University of Rochelle in France studied the diet of short-beaked common dolphins (Delphinus delphis), the most abundant species of dolphin living in the warm off-shore waters of the Atlantic.

They compared the range of fish species found in the stomachs of dolphins accidentally caught in tuna drift nets off the Bay of Biscay, with the abundance of fish species in the sea, measured by trawling surveys.

The scientists found that the dolphins have a distinct palate.

Instead of eating more of the most common species, which would be expected if dolphins feed opportunistically, the dolphins carefully selected which fish to consume.

For example, the dolphins regularly ignored fish that contained less than 5kJ per gram of energy.

These included the most abundant fish, a alepocephalid scientifcally named Xenodermichtys copei, which has 2.2kJ per gram of energy, and fish such as the Bean's sawtooth eel (Serrivomer beanii) which contains 2.1kJ per gram, and the Boa dragonfish (Stomias boa ferox) which has 2.8kJ per gram.

The dolphins mostly ate two species of less common lantern fish, the Kroyer's lanternfish (Notoscopelus kroeyeri) which contained 7.9kJ per gram and the Glacier lanternfish (Benthosema glaciale) which has 5.9kJ per gram.

Other research backs this finding, suggesting that striped dolphins (Stena coeruleoalba) also appear to prey on high quality Kroyer's lanternfish more often than other species.

Cold-blooded menu

Dolphins probably need to feed on high-energy fish to fuel their own energetic lifestyle, as warm-blooded social animals that range widely and can swim at high speeds.

Other large cold-blooded ocean predators, such as blue sharks (Prionace glauca) or swordfish (Xiphias gladius) rarely appear to take high-quality fish, preferring to dine on larger, leaner prey.

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